linux-kernel - Re: [PATCH v11 6/9] x86/kexec: Disable LASS during relocate kernel

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Message-ID: <54df1d32-d26b-4fa3-81cc-26f3e6402574@intel.com>
Date: Fri, 31 Oct 2025 10:14:29 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Sohil Mehta <sohil.mehta@...el.com>, x86@...nel.org,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>
Cc: Jonathan Corbet <corbet@....net>, "H . Peter Anvin" <hpa@...or.com>,
 Andy Lutomirski <luto@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, Ard Biesheuvel <ardb@...nel.org>,
 "Kirill A . Shutemov" <kas@...nel.org>, Xin Li <xin@...or.com>,
 David Woodhouse <dwmw@...zon.co.uk>, Sean Christopherson
 <seanjc@...gle.com>, Rick Edgecombe <rick.p.edgecombe@...el.com>,
 Vegard Nossum <vegard.nossum@...cle.com>,
 Andrew Cooper <andrew.cooper3@...rix.com>,
 Randy Dunlap <rdunlap@...radead.org>,
 Geert Uytterhoeven <geert@...ux-m68k.org>, Kees Cook <kees@...nel.org>,
 Tony Luck <tony.luck@...el.com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-efi@...r.kernel.org
Subject: Re: [PATCH v11 6/9] x86/kexec: Disable LASS during relocate kernel

On 10/29/25 14:03, Sohil Mehta wrote:
> The relocate kernel mechanism uses an identity mapping to copy the new
> kernel, which leads to a LASS violation when executing from a low
> address.
> 
> LASS must be disabled after the original CR4 value is saved because
> kexec paths that preserve context need to restore CR4.LASS. But,
> disabling it along with CET during identity_mapped() is too late. So,
> disable LASS immediately after saving CR4, along with PGE, and before
> jumping to the identity-mapped page.

It's not great when we have to thread the needle like this, but I don't
have any better ideas:

Reviewed-by: Dave Hansen <dave.hansen@...ux.intel.com>